Rhenium is a precious metal which is extremely costly. Rhenium has a wide variety of uses. One use for rhenium is as a promoter on an epoxidation catalyst.
U.S. Pat. No. 5,739,075 (the '075 patent) describes a process for the preparation of silver-containing catalysts suitable for the preparation of ethylene oxide. The process deposits a number of materials, including silver and a promoting amount of rhenium, on a porous refractory support. The '075 patent also describes rhenium compounds that may be solubilized for deposition on a support. See col. 13, 11. 2-32.
Unfortunately, epoxidation catalysts are subject to a performance decline, which represents itself by a loss in activity of the epoxidation catalyst and a loss in selectivity in the formation of the desired olefin oxide. In response to the loss of activity, the epoxidation reaction temperature may be increased such that the production rate of the olefin oxide is maintained. The operation of commercial reactors is normally limited with respect to the reaction temperature. When the applicable temperature limit has been reached, either the production rate of the olefin oxide is reduced or the production of the olefin oxide is interrupted to exchange the existing charge of epoxidation catalyst for a fresh charge. A fresh charge of catalyst comprising rhenium is very expensive.
Some efforts have been made to regenerate epoxidation catalysts. For example, U.S. Pat. No. 4,529,714 describes a process for regenerating silver-containing carrier catalysts used in the preparation of ethylene oxide which comprises treating a deactivated catalyst with a solution comprising a potassium, rubidium, or cesium component and a reducing agent.
A need exists for methods which reduce the cost of using rhenium as a promoter component on epoxidation catalysts.